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Old Feb 1, 2005, 7:55 AM   #1
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A "silly" question for someone new to photography. What is the difference between a lens that offers an F2.8 aperture over its entire range of focal lengths versus a "regular lens"?

Sigma 18-50mm F2.8 EX DC
Sigma 18-50mm F3.5-5.6 DC

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Old Feb 1, 2005, 8:58 AM   #2
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The difference is that the lens can have the max aperture throughout its entire range. Oh, did that answer beg the question? Sorry.

Basically, lenses with a fixed maximum aperture are better because

(1) having a wider maximum aperture allows more flexibility as your "wide open" shooting can allow more light in to the sensor (or film), allowing for better depth-of-field and faster shutter speeds, even at a higher focal length. When using a "regular" lens, if you zoom the lens in (to a higher focal length) you increase its length and thus increase the ratio between the length and the "opening" in the lens. Thus, the f-stop number is higher. The result is that less light can pass through the shutter. To compensate for that, you often have to slow down the shutter speed (to allow more light in over a given time) or increase the ISO (to make the sensor more sensitive to light, but also creating more "noise" or grain).

(2) Many of these lenses are often built better and have better optics. Also, they are not prone to lens creep (having the lens slide out or in on its own due to gravity when you're pointing the lens up or down)
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Old Feb 1, 2005, 12:09 PM   #3
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In general, constant appature lenses (everything else being equal) are higher end and more expensive than the variable ones. A constant appature f 2.8 lens will probably be pretty big and heavy.
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Old Feb 1, 2005, 2:13 PM   #4
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A fixed max aperture (the first in your list) will cost more and be heavier than the other too.

Is it worth it? For many people, yes.

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Old Feb 4, 2005, 2:49 PM   #5
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Some cameras like the 20D have one or more high precision focus points (the 20D has one in the center) that only operate as high precision with lenses off2.8 or faster. These high precision focus points stay active with a constant aperture F2.8 lens. While they revert to standard precision on lenses that slide to smaller F stop as you zoom.

You also get a brighter image in the viewfinder with faster lenses.

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Old Feb 6, 2005, 10:08 AM   #6
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How do u know if the lens has a fixed aperture?
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Old Feb 6, 2005, 8:27 PM   #7
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Check the manufacturer's site or any other site that has product info. Typically the name will state whether or not it is constant aperture. For instance, in a couple canon lenses I have: Canon 17-40L 4.0 has a constant 4.0 aperture. The Canon 28-135 3.5-5.6 has a 3.5 at the shortest zoom but a 5.6 at it's farthest reach.
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